The dramatic retreat of perennial Arctic sea ice has been a wake-up call to the climate community that climate change may not necessarily be slow and steady nor its impacts only of consequence in the far-off future. The newly revealed open waters of the Arctic Ocean and the collapse of warm-season snow cover are known to have profound impacts on the energy balance of the Arctic. And just as heating anomalies in the tropics can influence weather around the globe, large heating anomalies in the Arctic basin may have ripple effects at lower latitudes, especially across the industrialized countries and population centers of the Northern Hemisphere (NH).

The Arctic has warmed more than twice as fast as the global average, a phenomenon known as Arctic amplification (AA). These profound changes to the Arctic system have coincided with a period of ostensibly more frequent events of extreme weather across the NH mid-latitudes, including extreme heat and rainfall events and recent severe winters. Though winter temperatures have generally warmed since 1960 over mid-to-high latitudes, the acceleration in the rate of warming at high latitudes relative to the rest of the NH started approximately in 1990. Trends since 1990 show a cooling trend over the NH continents especially Northern Eurasia.

The possible link between Arctic change and mid-latitude climate and weather has spurred a rush of new observational and modeling studies. A number of workshops held during 2013-2014 have helped frame the problem and laid the groundwork to call for continuing and enhanced efforts for improving our understanding of Arctic-mid-latitude linkages and its attribution to occurrence of extreme climate and weather events. Although these workshops have outlined some of the major challenges and provided broad recommendations, no efforts had been made to synthesize the diversified research results to identify where community consensus and gaps exist.

Building upon findings and recommendations of the previous workshops, the US CLIVAR Working Group on Arctic Change and Possible Influence on Mid-latitude Climate and Weather convened an international workshop at Georgetown University in Washington, DC, on February 1-3, 2017, to assemble experts across the fields of atmosphere, ocean, and cryosphere sciences to assess the rapidly evolving state of understanding and to identify consensus on knowledge and gaps in research, and to develop specific actions to accelerate progress within the research community. With more than 100 participants, the workshop was the largest and most comprehensive gathering of climate scientists to address the topic to date. The meeting was truly international, with nearly a third of the scientists attending from twelve countries outside the US: Canada, China, Denmark, Finland, France, Germany, Japan, Republic of Korea, The Netherlands, Norway, Sweden, and the UK. The organizers were pleased that one-third of the participating scientists were early career.

This workshop was sponsored by the four US CLIVAR funding agencies: NASA, NOAA, NSF, and DOE with matching sponsorship by the NSF Arctic Natural Sciences Program and the World Meteorological Organization’s Polar Prediction Project. Eleven program managers from the US sponsoring agencies attended, reflecting the broad interagency interest in the topic.

The workshop agenda purposefully organized invited summary presentations into two plenary sessions, with corresponding poster and discussion sessions. The first focused on the observational analysis and the second on modeling studies, with both aimed at illuminating the current understanding of changes in the Arctic and their linkages to mid-latitude extreme climate and weather. Breakout sessions on the second day provided the opportunity for smaller, focused group discussion of (i) basic science questions on mechanisms and (ii) modeling experiments of Arctic amplification and its mid-latitude linkages. Key findings and recommendations that emerged from plenary and breakout discussions are summarized below. A more detailed white paper report is in preparation for publication by US CLIVAR in spring 2017.

Workshop Findings

Arctic rapid change is clearly evident in the observations and is simulated and projected by the GCM models. AA or temperature trend divergence between high- and mid-latitudes is prominent and even accelerating. AA is attributed to sea ice and snow decline (regionally and seasonally varying), however other factors can also greatly contribute to AA, including increased downwelling longwave radiation from greenhouse gases; greater water vapor concentrations from local and remote sources; increasing ocean heat content; local and hemispheric atmospheric circulation changes; increased poleward heat transport in the atmosphere and ocean; and poleward moisture transport and cloud radiative forcing. From discussions, it became evident that our understanding of AA is incomplete, especially the different relative contributions of the various radiative, thermodynamic, and dynamic processes.

Arctic mid-latitude linkages – Focusing on seasonal and regional linkages and addressing sources of inconsistency and uncertainty among studies

It is likely that rapid Arctic change is contributing to changes in mid-latitude climate and weather as well as occurrences of extreme events, but how significant the contribution is and what mechanisms are responsible are less well understood. Based on the consideration of observational and modeling studies, the workshop participants identified a list of proposed physical processes or mechanisms that may play an important role in linking Arctic change to midlatitude climate and weather. The list, ordered from high to low confidence, includes: increasing geopotential thickness over the polar cap; weakening of the thermal wind; modulating stratosphere-troposphere coupling; exciting anomalous planetary waves or stationary Rossby waves in winter and transient synoptic waves in summer; altering storm tracks and occurrence of blockings; and increasing frequency of occurrence of wave resonance. The pathway considered most robust is the low Barents-Kara Sea ice resulting in a northwestward expansion of the Siberian high leading to cold Eurasian winters. Complicating our ability to quantify these mechanisms are influences of or modulation by tropical and extratropical forcing – e.g., ENSO, AMV, PDO, MJO – and uncertainties due to metrics, analysis, and modeling approaches employed.

Opportunities and Recommendations

An important goal of the workshop, as described above - to make greater progress towards consensus understanding and identification of knowledge gaps than previously accomplished - was achieved. Based on the workshop findings, the participants identified specific opportunities to utilize observations and models, particularly together, to enable and accelerate progress in determining the mechanisms of Arctic rapid change and its mid-latitude linkages.

Observations

Due to the remoteness and harsh environmental conditions of the Arctic, in situ observational time series are highly limited spatially and temporally, although a few of drifting ship or buoy observations along sea ice motion trajectories can be traced back to the late 19th century. Satellite remote sensing estimates of sea ice concentration and a number of other limited atmospheric parameters, such as clear-sky surface temperature, cloud cover, provide an improved spatial coverage but the data is only available since the late 1970s. In mid-latitudes, the times series of surface and atmospheric properties are much longer, spanning the twentieth century and enabling the analysis of variability and changes in the large-scale atmospheric circulation and extremes in climate and weather. In particular, model- and data-assimilation-technology-based reanalysis products provide the best estimate of a three-dimensional state of the atmosphere, which can be extended back to the late 19th century and make it possible to analyze comprehensive features of the atmospheric circulation and weather patterns. However, their reliability for evaluating variability and changes needs to be determined, in particular considering the lack of observations in the Arctic.

Recommendations emerged from the workshop to expand the observational datasets and analyses approaches of Arctic change and mid-latitude linkages: to synthesize new Arctic observations to provide the best high-resolution estimate of the atmospheric state for better understanding sea ice and ocean surface processes; to create physically-based sea ice/ocean surface forcing data sets available to investigate Arctic-midlatitude linkages; to systematically employ proven and new metrics to identify forced signals of atmospheric circulation from natural variability; to analyze paleoclimate data and new observational datasets that span most of the past century, including reanalysis and sea ice; and to utilize new observational analysis methods (e.g., fluctuation-dissipation analysis, causal effect networks) that extend beyond correlative relationships to establish causal links between forcing and response.

Model experiments

Participants acknowledged that models provide the primary tool for gaining a mechanistic understanding of variability and change in the Arctic and at mid-latitudes. Uncertainties in model results arise from experiment design, forcing prescription, and model systematic errors. Coordinated modeling studies should include approaches using a hierarchical of models from a conceptual or simple component or coupled models to complex atmospheric climate model (AGCM) or fully coupled Earth system models (CESM). Coordinated model intercomparison project (MIP) experiments, utilizing multiple models with each adhering to a common set of forcings, simulation protocols, and specified output parameters, will enable a more systematic evaluation of mechanisms and pathways linking Arctic and midlatitude climate and weather, as well as associated feedback processes.

The modeling breakout group proposed the creation of a modeling task force to coordinate MIP experiments. A multi-tiered set of MIP experiments is envisioned, drawing from the initial planning and discussions of the US CLIVAR Working Group and planned modeling element of the European Horizon 2020 projects (APPLICATE, Blue Action, and PRIMAVERA). Tier one experiments would consist of two fast-track sets of AMIP-like simulations that can be conducted by different groups and made available to the community for analysis relatively quickly. Fast-track #1 would exploit the CMIP6 AMIP (from 1979-present) as the control run, and modeling groups would then execute two sets of sensitivity experiments – one with climatological sea ice and the other with climatological sea surface temperatures (SSTs) – to evaluate the atmospheric response to recent AA. For fast-track #2, modeling groups would run AMIP-like control simulations with observed climatological sea ice and SST and then three different time slice experiments using modeled sea ice and SSTs patterns from the past (pre-industrial), the present (transient runs), and future (pattern under +2°C warming). It is planned that protocols for these fast-track experiments will be determined by June 2017. Future tier two and tier three experiments using more complex (uncoupled and/or coupled) models and designed to address specific hypothesized mechanisms and pathways will be scoped by the Working Group and task team, as well as interested groups over the next year. Model simulation output from the experiments is planned to be served through the Earth System Grid, making it available to the broader community for evaluation of the proposed mechanisms linking changes in the Arctic to mid-latitudes.